The present invention relates generally to nuclear power plants and, more specifically, to a hybrid power plant combining a nuclear power plant or a biomass fired power plant with a fossil fuel fired power plant to provide improved efficiencies and reduced emissions.
The vast majority of energy production in the world comes from one of four non-renewable sources: coal, gas, petroleum or nuclear. According to the most recent data (CY 2006) from the International Energy Agency, 85% of electricity was generated from nuclear (23.2%) and combustibles (61.8%), while hydroelectric was 13.4% and other renewables was 1.6%. Each of these sources has its strengths and weaknesses. US only data from the US Department of Energy breaks down combustibles as coal 49.7%, natural gas 18.7% and petroleum 3%. Petroleum is almost always reserved for transportation and is not normally used in electrical power generation. Natural gas is used, but because of its cost is normally only used to power peak period surge capacity. This leaves nuclear and coal fired plants to provide base load and the majority of electricity in the world.
Coal currently provides the vast majority of base load electrical generating capacity and about half of all capacity, but its use is coming under heavy attack for pollution concerns and especially the “greenhouse gas” emissions of carbon dioxide. Nuclear's use has been limited by it high costs of production, largely driven by the very low thermal efficiency of its steam cycle that requires a very large reactor relative to the amount of electricity that can be generated by its low temperature saturated steam. Biomass has been investigated, but because of the high water content and low energy density it is not possible to achieve combustions temperatures comparable to coal combustion. This results in lower efficiencies from low temperature saturated steam, much like those that limit nuclear power.
Current applications for addressing environmental and efficiency issues center around multiple use facilities. These facilities use a single source of energy to satisfy several needs, many by exploiting synergies between emissions control and unused waste energy or combustion products. This patent proposes a more effective approach to the multiple use facility by using more than one energy source in a hybrid power plant to use the strengths of the separate technologies to address the accompanying weaknesses. A hybrid plant combining the existing technologies of nuclear power plants or biomass power plants interconnected to a modified coal plant would result in a total thermal process that would have a greatly improved thermal cycle, thereby increasing electrical output to nearly double from the same sets of inputs compared to ‘stand-alone’ configurations, thereby dramatically lowering cost, pollution and carbon dioxide emissions relative to two stand alone plants of these designs.
Coal-fired fossil fuel plants generally operate at the highest levels of thermal efficiency, with electricity output to heat unit input fractions in the 30-45% range. This is accomplished through a three-step steam cycle. First, the feedwater to the boiler is pre-heated with the low temperature effluent combustion gasses extraction steam to increase the temperature from condenser temperature to approximately 450-500° F. Once the feed water is added to the boiler, it is heated and converted to saturated steam at temperatures of 500-600° F. Once the steam is formed in the boiler, it passes through superheat tubes in the hottest section of the effluent gas column where the steam is increased in temperature to 1100° F.-1200° F. This superheated steam is then passed through a series of high, intermediate and low pressure turbines where energy is extracted and electricity is produced by generators mechanically attached to the turbines. A final step in a coal-fired plant process for electricity generation is that the air being drawn into the firebox is passed through the lowest temperature effluent gasses to pre-heat the incoming air and increase the temperature of combustion.
A coal-fired plant is very efficient, but even in this type of plant most of the energy of combustion is lost. Of the 1512 BTUs required to heat a pound of ambient 140° F. (60° C.) feedwater to a pound of superheated steam at 1200° F. (650° C.), 1000 psi steam, 1014 BTUs or 67% of the input energy goes to converting the water to steam and cannot be recovered as electrical output. Approximately another 40 BTUs (about 3% of the total) are also un-recoverably lost in each cycle. The condensers downstream of turbines will operate at a vacuum, so that the steam will not reconvert to water at the normal 212° F. (100° C.) boiling point, but at a temperature of 140° F. (60° C.). However, this water will continue to cool to the temperature of the river or lake being used as the heat sink, and this heat will have to be replaced in the next cycle. Usable (available for conversion to electricity) energy can be extracted from the steam from 1200° F. (650° C.) to steam at 140° F. (60° C.). This means that less than one of every two tons of carbon dioxide that a coal powered plant emits to the atmosphere is ever used to produce electricity.
The use of biomass in place of coal in a boiler requires a configuration much like that of a pulverized coal boiler, although the operation of the plant is altered. While there is a ‘net zero’ carbon emission from these facilities, biomass has a lower energy density and flame temperature than coal when combusted under the same conditions. This reduces the amount of energy that can be imparted to the feed water, reducing the steam temperature to usually no more than 850° F. steam. Because of the lower operating temperature a lower operating pressure is used to increase cycle efficiency, so an operating pressure of 850 psi is assumed. This is a heat addition of 1317 BTUs per pound to ambient feedwater, of which about 1014 BTUs are lost due to the phase change from steam to water and other losses. This results in 77% of the energy not being available to produce electricity.
The current state of the art nuclear power plants (including pressurized light water reactors, boiling water reactors, and heavy water CANDU designs) are extremely stable, safe, and emission free. Their power output is extremely restricted, however, by the need to limit the maximum temperature in the reactor core to approximately 600° F. (boiling water reactors operate at lower core temperatures of about 540-550° F.) to prevent loss of coolant and damage to the fuel elements. This results in a vastly oversized reactor plant and the wasting of a high percentage of the BTUs generated. This results in excessive thermal pollution—the localized heating of the bodies of water that serve as heat sinks for the condensers of the steam turbine units.
The nuclear power plant has only two of the three steps of the steam cycle. Essentially no superheat is added to the steam cycle as the water in the steam generator is already in contact with the hottest water to pass through the reactor. Methods exist to preheat the feedwater entering the steam generator, but this is done solely with extraction steam, requiring a higher steam flow rate for the same electrical output. The primary coolant water in contact with the reactor core heats to 600° F. before moving to the steam generator (the same function as the boiler in the coal-fired plant) and converting secondary water to steam at about 575° F. with an operating pressure of 400 psi to increase efficiency. This results in a steam cycle where only 1199 BTUs can be added to each pound of steam, yet the same 1014 BTUs are lost in changing the water to/from steam, so that fully 85% of the heat energy input can never be used in the creation of electrical energy. By combining the higher temperatures achievable in a coal furnace with the low temperature steam from a nuclear or biomass plant, a higher efficiency can be realized with fewer emissions compared to either design alone.
A search of prior art was conducted and the following related patents were discovered. None of these patents teach or suggest any method or device matching this invention.
U.S. Pat. No. 3,575,002 by Vuia was for a design that routed the saturated steam from a standard nuclear power plant through the superheater section of a fossil fuel furnace in a conventional power plant. While a feasible solution, a majority of the energy input to the system is from coal, as this is a full scale fossil fuel power plant with a slightly larger superheater section in the furnace. This design by Vuia proposes a design with two independent power plants in which the nuclear is assisted by the coal plant. In contrast this invention proposes a single integrated hybrid power plant that uses the energy from the coal only to add superheat to the steam, decreasing the amount of coal used to generate the same amount of energy.
U.S. Pat. No. 4,530,814 to Schluderberg uses the thermal energy from a fossil fired plant to produce steam. This steam is then routed through a moisture separator/reheater unit to add superheat to steam that has already been expanded through a high pressure turbine. This design uses the fossil fuel exclusively to add superheat to the nuclear process steam, but does so indirectly and only after the steam pressure has been lowered. In this design the power plant steam flows again remain separate and the coal plant only provides a reheat assist to the nuclear power plant, no energy is made available to preheat feedwater.
U.S. Pat. No. 5,361,377 to Miller describes the use of superheaters before the high pressure turbine and in the moisture separator/reheater section between turbines. The superheater described may receive energy either from fossil fuel combustion or steam from an adjacent fossil fuel plant. The description is unclear on how the superheater would be able to use either steam or fossil fuel. The design also fails to make full use of the exhausted flue gases to preheat feed water and combustion air, indicating that it is a small burner unit and not a full size coal burning furnace. This design appears to only pertain to an externally heated superheater on a nuclear power plant.
U.S. Pat. No. 5,457,721 to Tsiklauri uses a combined cycle system with the hot exhaust gases from a natural gas fired gas turbine unit heating feedwater and producing steam. The steam from this heat recovery steam generator is then used to superheat the steam from a nuclear powered steam generator. After the steam is expanded in the high pressure turbine, the two fluid streams are mixed and augmented by more steam from the heat recovery steam generator and used in the low pressure turbine. This use of a heat recovery steam generator decreases the efficiency of the system as opposed to using all the energy to add superheat. Mixing the steam from both sources decreases this efficiency loss, but would require stricter water chemistry controls.
U.S. Pat. No. 6,244,033 to Wylie uses the exhaust from a natural gas fired gas turbine unit to directly superheat the steam from a nuclear steam generator. It also makes use of the exhaust gases to preheat the feedwater and provides a supplemental fire unit to ensure there is sufficient energy to provide the superheat and preheat. Notable in this patent is that it specifies that superheat and preheat can be added by the use of additional natural gas heat addition alone if the gas turbine unit is not in operation. There is no provision for the use of coal in this patent, only more expensive natural gas.